1. Field of the Invention
The present invention relates to a process and a device for producing curved glass panes. The device includes a bending press, comprising an upper, solid-faced bending mold and a lower, horizontally movable frame-shaped mold, and a cooling station, in which the bent glass panes are cooled as they lie on the movable frame-shaped mold.
2. Description of the Related Art
Devices of this type are widely used for making monolithic safety glass panes of toughened glass for the automobile industry. In this case the cooling station contains blowing boxes, by means of which the glass panes are rapidly cooled by air over their entire surface at a high rate of cooling. By the accelerated cooling, all over the glass sheet except the region bordering the glass sheet, high compressive stresses result in the surface layers of the glass panes, and these are compensated by high tensile stresses in the core. The thickness stress distribution is isotropic, i.e., the integral of the stresses along the thickness is zero. In the region bordering the edges, the integral of the stresses is no longer zero but leads to a preponderance of compressive stresses known as edge stresses.
Devices of the aforementioned type may, however, be used also for fabricating curved automobile glass panes of laminated safety glass. Laminated safety glass panes consist of two relatively thin single glass sheets which, after they have been bent, are bonded together by a film of thermoplastics. The two single glass sheets may be bent individually or as a matching pair. The cooling of the glass sheets after bending takes place relatively slowly, because the glass sheets should be substantially free of stresses. This cooling is generally achieved in an annealing station in which occurs the relaxation of the internal stresses induced during the bending step. But this annealing does not prevent a more pronounced cooling of the edges in order of generating edge stresses, to increase the mechanical strength and the thermal resistance of the peripheral zone.
From DE 26 40 206 C2, it is known, for example, to cool the glass sheets with a thickness of 1.5 to 2.5 mm in the peripheral zone at a rate of cooling of 85 to 150 degrees/min, in order to thereby generate edge compressive stresses of 1,960 to 3,430 N/cm.sup.2 in the peripheral region.
The edge stresses generated in the peripheral region are compensated by tensile stresses, which form in the zone adjacent to the compressively stressed peripheral zone. These tensile stresses in the zone adjacent to the peripheral zone can, in the case of laminated glass panes, lead to a weakening of the glass pane and for this reason must not exceed a specified maximum value.
Due to an inherent limitation of the edge stresses, the limitation of the tensile stresses in the edge region of the glass sheets does not present any difficulties in the bending method most commonly used at present for laminated glass, in which the pair of single glass sheets, resting on a frame-shaped bending mold, are moved through a bending furnace, in which they become curved under the effect of their own weight. It has been found, however, that it is relatively difficult to produce curved glass sheets for laminated glass with the desired tensile stress state in the edge region by means of a pressing bending device of the initially named type.